Low-pressure Mercury Vapor Discharge Lamp and Ultraviolet-ray Irradiating Apparatus and Method Using the Same
The present invention relates to a low-pressure mercury vapor discharge lamp with a relatively high electric power density and a relatively long effective light emission length which is suitable for use in purification, sterilization, disinfection or the like of water by radiation of ultraviolet rays, as well as an ultraviolet-ray irradiating apparatus and method using such a low-pressure mercury vapor discharge lamp.
Ultraviolet rays of a short wavelength range have been used for sterilization, decomposition of toxic organic substances, etc., and low-pressure mercury vapor discharge lamps have heretofore been known as sources for generating ultraviolet rays having a wavelength, for example, of 185 nm or 254 nm. Generally, the low-pressure mercury vapor discharge lamps contain a rare gas, such as argon (Ar) along with a superfluous amount of mercury, and a vapor pressure (vaporization amount) of the mercury varies in response to a temperature of a coldest portion within the discharge lamp. Radiation efficiency of the ultraviolet rays is closely related with the mercury vapor pressure; for example, the 254 nm ultraviolet rays present a highest radiation efficiency at a vapor pressure of about 6xc3x9710xe2x88x923 torr and at a 40xc2x0 C. temperature. At 70xc2x0 C., the vapor pressure of the ultraviolet rays rises to about 5xc3x9710xe2x88x922 torr, and the radiation efficiency decreases by more than 20%. For this reason, the low-pressure mercury vapor discharge lamp is normally designed such that the temperature during operation is held at and around 40xc2x0 C. In recent years, attempts have been made to increase the density of electrical energy input to the discharge lamp (lamp input density) for an enhanced processing capability of the discharge lamp; in this case, the operating temperature would exceed 40xc2x0 C., so that there has been employed an approach of enclosing the mercury in an amalgam state. This approach comprises alloying the mercury with another metal, such as bismuth (Bi), tin (Sn) or indium (In) and placing the resultant alloy within the discharge lamp to thereby suppress the mercury vapor pressure during high-temperature operation. Exemplary comparison between a vapor pressure curve of an indium-bismuth amalgam and a vapor pressure curve of mercury (pure mercury) is given in FIG. 5.
FIG. 4 shows an example of a conventional low-pressure mercury vapor discharge lamp. Here, reference numeral 1 represents a light-emitting tube bulb formed of quartz glass, which has opposite ends hermetically closed by glass stems 2a and 2b. Reference numeral 4 represents an indium-bismuth amalgam fixed on the glass stem 2a. Reference numerals 21a and 21b represent a pair of filaments, which are each coated with a barium-oxide (BaO)-based thermoelectronic substance in order to permit a smooth electric discharge. These filaments 21a and 21b are retained on the respective glass stems 2a and 2b, and are electrically connected, via lead wires 22a, 22b and 22c, 22d, to terminals 31a, 31b and 31c, 31d, respectively, of metallic caps or bases 3a and 3b. In the light-emitting tube bulb 1, there is also contained an appropriate amount of argon (Ar) gas. Once the low-pressure mercury vapor discharge lamp is turned on by being connected to a predetermined power supply, electric discharge is produced between the filaments 21a and 21b, so that the mercury vapor is increased by a heat resulting from the electric discharge (discharge heat) and the vaporized mercury atoms are excited to emit ultraviolet rays.
Although the mercury vapor discharge lamp containing an amalgam has a great advantage of ensuring a high ultraviolet-ray radiation efficiency by suppressing the mercury vapor pressure during high-temperature operation, it would present significant inconveniences or disadvantages due to the fact that the mercury vapor pressure is suppressed not only during the high-temperature operation but also in low-temperature conditions prior to the turning-on or lighting-up of the lamp. One of such inconveniences is that the discharge lamp can not be readily activated because a high voltage is required to start the electric discharge. Normally, the temperature within the light-emitting tube bulb prior to the lighting-up is substantially equal to a temperature of an atmosphere in which the lamp is placed. For example, in a situation where the temperature of the atmosphere is 20xc2x0 C., there exists a mercury vapor pressure of about 1.2xc3x9710xe2x88x923 torr in a discharge lamp containing a normal form of mercury (pure mercury), and the necessary discharge-starting voltage can be lowered greatly by the Penning effect produced by the mercury vapor pressure and argon gas, so that the electric discharge can be initiated smoothly. By contrast, in a discharge lamp containing an amalgam, the mercury vapor pressure prior to the lighting-up is suppressed below 1/10 of that in the above-mentioned mercury-containing discharge lamp, which would lessen the Penning effect and hence raise the necessary discharge-starting voltage level. Thus, activating the amalgam-containing discharge lamp would require a higher discharge-starting voltage than required for activation of the traditional-type discharge lamp.
Another inconvenience presented by the amalgam-containing discharge lamp is a slow rise in the light amount of the emitted ultraviolet rays. It is considered that a primary cause of such a slow rise in the light amount is a synergism of several factors, such as: insufficient emission of ultraviolet rays immediately after the lighting-up due to an inherently small amount of mercury vapor within the discharge lamp; an insufficient lamp input immediately after the lighting-up because of the small mercury vapor amount; a hard-to-warm tendency of the discharge lamp due to an insufficient discharge heat resulting from the insufficient lamp input immediately after the lighting-up; and even slower evaporation of the mercury from the amalgam due to the hard-to-warm tendency of the discharge lamp.
Even in the discharge lamp containing the mercury in an amalgam state, these inconveniences would not lead to practical problems as long as the lamp""s effective light emission length (which equals a length between the filaments) is relatively short. Because, the discharge lamps with a short effective light emission length can be activated with a relatively low discharge-starting voltage and can be filled with mercury vapor at a rapid speed. Further, in the discharge lamp with a low lamp input density, presence of the above-mentioned inconveniences is not even considered to be problematic, because there is no absolute necessity to contain the mercury in an amalgam state. However, the above-mentioned inconveniences would become serious problems with such an elongated, high-density discharge lamp that is often required in the field of purification processing by ultraviolet rays. Namely, in recent years, there has been an increasing demand for further enhanced processing capabilities in the field of the purification processing based on use of ultraviolet rays, and therefore a discharge lamp with a longer effective light emission length as well as a higher lamp input density has become necessary for an increased processing capacity. In such a discharge lamp with a longer effective light emission length, the above-mentioned inconveniences would become significant problems to be properly overcome since the necessary discharge-starting voltage has to increase as the effective light emission length increases and the increased effective light emission length results in a greater time lag until the mercury vapor fills the entire interior of the discharge lamp. As an example of such a discharge lamp, there is currently being used a high-density discharge lamp with a lamp input density exceeding about 1 W/cm. With this type of high-density discharge lamp, the temperature during the lighting-up operation would become very high so that there arises a need to employ an amalgam with a further suppressed mercury vapor pressure. If such an amalgam with a further suppressed mercury vapor pressure is employed, the necessary discharge-starting voltage has to increase, which would result in a slower rise in the light amount of the ultraviolet rays.
According to the concept of the conventionally-known technique, an even higher voltage is required to start the electric discharge in the above-mentioned type of elongated, high-density discharge lamp. However, the even higher discharge-starting voltage is undesirable, because the sterilization technique and technique of decomposing toxic organic substances using ultraviolet rays are often employed in applications, such as water purification processing, where water is processed with the ultraviolet rays and the excessive discharge-starting voltage could cause a breakdown (electric discharge through an insulator) of related equipment. Further, from a viewpoint of environmental protection, it is absolutely necessary to avoid a spill of raw water having been processed insufficiently due to the slow rise in the light amount of the ultraviolet rays.
It is therefore an object of the present invention to provide an improved low-pressure mercury vapor discharge lamp with a high lamp input density and long effective light emission length which requires only a low discharge-starting voltage such that it can light up without a very high voltage having to be applied thereto and which permits a rapid rise in a light amount of ultraviolet rays. It is another object of the present invention to provide an ultraviolet-ray irradiating apparatus using such an improved low-pressure mercury vapor discharge lamp.
According to one aspect of the present invention, there is provided a low-pressure mercury vapor discharge lamp which has an effective light emission length not shorter than 40 cm and a lamp input density, per unit length of the effective light emission length, not lower than 0.9 W/cm and which contains at least mercury as a light-emitting metal and an activating rare gas. The low-pressure mercury vapor discharge lamp of the invention is characterized in that the mercury is provided in an amalgam with another metal and that the discharge lamp further includes a thin coating formed on a glass inner surface thereof for trapping a minute amount of the mercury. During lighting-up operation of the low-pressure mercury vapor discharge lamp thus arranged, an appropriate amount of the mercury, corresponding to a temperature of the amalgam, vaporizes, which contributes to a higher efficiency of ultraviolet ray emission. Once the low-pressure mercury vapor discharge lamp is turned off (caused to stop illuminating), part of the mercury vapor returns to the amalgam while the remaining part of the mercury vapor present in the vicinity of the mercury-trapping thin coating is drawn, as grains of mercury, onto the thin coating on the glass inner surface of the discharge lamp. Thus, when the discharge lamp is turned on next, only a low discharge-starting voltage is required because of presence of mercury vapor from the grains of mercury sticking to the thin coating. In addition, the presence of the mercury vapor at the lighting-up of the discharge lamp achieves a quick rise in the light amount of the ultraviolet rays. Consequently, the low-pressure mercury vapor discharge lamp of the present invention can effectively avoid the inconveniences of the conventionally-known technique.
Particularly, with the conventionally-known low-pressure mercury vapor discharge lamp whose effective light emission length is 40 cm or more, the necessary discharge-starting voltage would exceed 1,000 V, and thus even more stringent safety would be required as specified by the technical standards for electric facilities and equipment, with the result that the discharge lamp tends to become more expensive. However, the present invention can eliminate such a problem because it can greatly lower the necessary discharge-starting voltage as compared to the conventional discharge lamp. Thus, the present invention achieves great benefits when the basic principles thereof are applied to a low-pressure mercury vapor discharge lamp whose effective light emission length is not shorter than 40 cm. Further, where the lamp input density is 0.9 W/cm or more, it would become difficult to attain an appropriate coldest temperature within the discharge lamp unless the mercury is contained in an amalgam state, even when the discharge lamp is lit up under low-temperature conditions with an atmosphere temperature of about 10xc2x0 C. In such a case, the inconveniences as discussed earlier would be encountered. However, the present invention can provide effective solutions to the inconveniences and therefore achieves great benefits when the basic principles thereof are applied to a low-pressure mercury vapor discharge lamp whose input density is not lower than 0.9 W/cm.
In a preferred embodiment of the present invention, the thin coating for trapping a minute amount of the mercury includes, as its main ingredient, an oxide of at least one metal selected from a group consisting of aluminum (Al), silicon (Si), calcium (Ca), magnesium (Mg), yttrium (Y), zirconium (Zr) and hafnium (Hf). The oxide of each of these metals has a good heat resistance and chemical stability and thus can effectively function as the mercury-trapping thin coating.
Further, according to the present invention, the amalgam may be secured to one or more locations of the glass inner surface facing the discharge space of the low-pressure mercury vapor discharge lamp. By the amalgam being thus secured to the glass inner surface facing the discharge space, the amalgam is exposed directly to the discharge space so that the temperature of the amalgam can increase relatively rapidly after the discharge lamp is turned on or lit up, which can promote vaporization of the mercury from the amalgam and thus even further promote the quick rise in the light amount of the ultraviolet rays.
Further, the present invention provides an ultraviolet-ray irradiating apparatus which is characterized by using the above-mentioned inventive low-pressure mercury vapor discharge lamp, as an ultraviolet-ray emitting source, to irradiate ultraviolet rays onto an object to be sterilized or disinfected. Because the inventive low-pressure mercury vapor discharge lamp can be activated with a low discharge-starting voltage and achieves a quick rise in the light amount of the ultraviolet rays and because it is designed as a high-density and elongated discharge lamp (with the lamp input density of 0.9 W/cm or more and the effective light emission length of 40 cm or more), the ultraviolet-ray irradiating apparatus using the inventive low-pressure mercury vapor discharge lamp can work with extremely high performance and reliability.